The Senate will vote shortly after 8 p.m. to begin what is expected to be a lengthy, no-holds barred, bare-knuckled debate on Majority Leader Harry Reid's $848 billion, 2,074-page bill in a rare Saturday session scheduled to last into the evening.

WASHINGTON -- Senate Democratic leaders lined up all 60 members of their caucus Saturday to support advancing legislation on a health care overhaul, as Republicans maintained unanimous opposition toward the bill that would achieve President Obama's top domestic priority.

The Senate is scheduled for a key procedural vote shortly after 8 p.m.. If passed, the vote would set the stage for a lengthy, no-holds-barred debate on Majority Leader Harry Reid's $848 billion, 2,074-page bill, though that debate won't get started until after the Senate's Thanksgiving break.

The rare Saturday session amounted to a first round in the fight to pass the bill in the full Senate, with the remaining Democratic holdouts announcing they would support at least the measure to open debate on the bill, avoiding an early knockout by Republicans.

Sens. Blanche Lincoln of Arkansas and Mary Landrieu of Louisiana waited until Saturday to say they would vote yes for a floor debate. Sen. Ben Nelson of Nebraska announced Friday his support for moving the bill forward.

All three cautioned that their votes to start debate should not be construed a support for the bill in its current form.

"It is a vote to move forward to continue the good and essential and important and imperative work that is under way," Landrieu said on the Senate floor. "I've decided that there's enough significant reforms and safeguards in this bill to move forward but more work needs to be done."

Lincoln said she still would support a filibuster if the so-called "public option," a government-run insurance plan, remains in the health care bill.

"I along with others expect to have legitimate opportunities to influence the health care reform legislation that is voted on by the Senate later this year or early next year," she said.

While the vote is only a procedural one, Republicans haven't backed down.

"This is a vote about whether or not you want to fundamentally change the way health care is delivered in this country in a way which massively expands the size of government, the role of government and significantly increases the tax burden, especially for small businesses and cuts Medicare by a dramatic amount of money," Sen. Judd Gregg, R-N.H., told Fox News before Saturday's session began.

Senate Minority Leader Mitch McConnell called the bill "monstrous" and, citing the Congressional Budget Office, said it would not bring down costs.

Democrats said their legislation could make historic and necessary improvements in the country's social safety net.

"Prices of health care are marching relentlessly upwards, and so too many people don't have coverage," said Sen. Byron Dorgan, D-N.D. "The purpose of all of this is to try to get a handle on it somehow."

Most Americans would be required to purchase insurance under Reid's legislation, and billions in new taxes would be levied on insurers and high-income Americans to help extend coverage to 30 million uninsured. Insurance companies would no longer be allowed to deny coverage to people with medical conditions or drop coverage when someone gets sick.

Republicans used their weekly radio and Internet address to slam the legislation, calling it a government takeover of health care that would increase taxes and raise medical costs.

"This 2,000-page bill will drive up the cost of health care insurance and medical care, not down," Sen. Mike Crapo, R-Idaho, said in the address. "This is not true health care reform, and it is not what the American people want. This bill will result in higher premiums and higher health care costs for Americans -- period."

The White House issued a statement late Friday praising the Senate measure. "This bill provides the necessary health reforms that the administration seeks -- affordable, quality care within reach for the tens of millions of Americans who do not have it today, and stability and security for the hundreds of millions who do," the statement said.

The action in the Senate comes two weeks after the House approved a health overhaul bill of its own on a 220-215 vote. After the vote Saturday night, senators will leave for a Thanksgiving recess. Upon their return, assuming Democrats prevail on the vote, they will launch into weeks or more of unpredictable debate on the health care bill, with numerous amendments expected from both sides of the aisle and more 60-vote hurdles along the way.

Senate leaders hope to pass their bill by the end of the year. If that happens, January would bring work to reconcile the House and Senate versions before a final package could land on Obama's desk.

The bills have many similarities, including the new requirements on insurers and the creation of new purchasing marketplaces called exchanges where self-employed individuals and small businesses could go to shop for and compare coverage plans. One option in the exchanges would be a new government-offered plan, something that's opposed by private insurers and business groups.

Differences include requirements for employers. The House bill would require medium and large businesses to cover their employees, while the Senate bill would not require them to offer coverage but would make them pay a fee if the government ends up subsidizing employees' coverage.

Another difference is in how they're paid for. The Senate bill includes a tax on high-value insurance policies that's not part of the House bill, while the House would levy a new income tax on upper-income Americans that's not in the Senate measure. The Senate measure also raises the Medicare payroll tax on income above $200,000 annually for individuals and $250,000 for couples. Both bills rely on more than $400 billion in cuts to Medicare.

The Senate bill was written by Reid in private negotiations with White House officials, combining elements of two committee-passed bills and making additional changes with an eye to getting the necessary 60 votes.

Along the way, Reid sweetened the pot for individual senators, adding federal funds for Louisiana and agreeing to support an amendment written by Sen. Ron Wyden, D-Ore., that would expand eligibility for the purchasing exchanges.